Electronic projection systems have been developed or commercialized using a wide range of modulation technologies. The various light modulation approaches used include deflection, diffraction, blocking or absorption, scattering, and rotation of polarization states, for example. Imaging devices using Liquid Crystal Devices (LCDs), for example, directly modulate the polarization state of incident light within each color channel in a pixel-by-pixel fashion. Imaging devices that use digital micro-mirror devices, include Digital Light Processor (DLP) devices from Texas Instruments Inc., Dallas, Tex., for example, consisting of micro-mirror arrays that deflect light in a pixel-by-pixel fashion.
Each particular light modulation technology has characteristic strengths and weaknesses. For example, in systems that modulate polarization, contamination can occur between on- and off-states due to inadvertent phase shifting, skew ray or large angle cross-talk, or stress birefringence. In alternate systems that modulate light by deflection or diffraction, angular scattering can cause re-direction of light beams into the imaging path. Often, the characteristic disadvantages of one type of light modulation are not factors with another type.
Polarization effects, however, impact numerous light modulation approaches. Polarization is used, for example, to enable 3D or stereo projection in high-end DLP-based systems. DLP projectors adapted for stereo projection using Real-D (Beverly Hills, Calif.) technology use a polarization rotator to switch between polarization states for right- and left-eye images, as perceived by viewers wearing polarization discriminating glasses. The polarization contrast required for 3D projection in DLP based systems is relatively modest (˜400:1), but ghost images can occur if the contrast is too low. Polarized light sources such as visible light emitting lasers are also being used for image projection systems, with or without 3-D projection. Taken together, the polarization sensitivity of a projector, relative to color or contrast artifacts, even in cases where the light modulation components themselves relatively insensitive to polarization, has increased compared to prior art systems.
Color combiners, and other types of optical components for separating and recombining spectral components and for redirecting and conditioning light within the projection apparatus, typically rely on multilayer thin-film optical coatings. Conventional coating designs provide various types of spectral filters with needed levels of reflectivity or transmission at selected wavelengths. However, conventional multilayer coating designs do not provide a highly uniform response to light having different polarization states. In the case of conventional reflective multilayer coatings, for example, there is higher reflectivity for light whose electric field vector oscillates perpendicularly to the plane of incidence (s-polarization) relative to the reflectivity for p-polarized light whose electric field vector oscillates parallel to the plane of incidence over the entire range of incidence angles. The reflectivity for p-polarized light reaches its minimum at the layer-specific Brewster angle. Consequently, noticeable amplitude splits for s- and p-polarized light can occur, particularly in the region about the Brewster angle. At lesser angles, spanning on-axis light, specular light, full-field light, or skew rays, the light efficiency varies with polarization and angle. As another example, if circularly polarized light falls onto a conventional, obliquely-positioned deflecting mirror formed from thin-film coatings, the p-component of the light is more strongly attenuated than the s-component of the light after the reflection. In a polarization sensitive projector, having a polarization analyzer such as a MacNeille prism or a wire grid polarizer, these light efficiency differences can introduce significant contrast or color shading errors. In nominally polarization-sensitive systems lacking a polarization analyzer, on the other hand, image artifacts from these polarization efficiency differences may go largely unnoticed.
Color combiners or other types of dichroic filters can also introduce phase changes for each polarization (Δφs and Δφp), as well as relative phase differences between the various polarization directions (Δφsp=Δφs−Δφp), meaning that the transiting light experiences differing amounts of rotation according to polarization and angle. As a result, the outgoing polarization state of the light is different from the incoming polarization state. These effects can be compounded in an optical system in which there are multiple reflections for polarized light, so that even slight phase shifts for polarized light on each of a number of surfaces can have an additive effect. Linear polarized light can easily become circularly or elliptically polarized, changing the polarization response as the light then transits downstream optics. For example, with conventional beam combiners, designed for optimal spectral efficiency performance, phase differences Δφsp between s- and p-polarized light often exceed ±20 degrees or more. Phase difference Δφsp as large as ±100 degrees or more can often be measured near edge transitions. By comparison, for polarization maintenance, best performance would be achieved with phase differences Δφsp at or near 0 degrees, or at least within no more than about ±10 degrees, without compromising image quality or brightness while attaining the phase performance.
Where thin-film surfaces handle the p- and s-polarized light differently, the effective contrast of a digital imaging system can be compromised due to light leakage; throughput also suffers accordingly. In systems that use polarized light for left- and right-eye image separation in stereoscopic (3D) imaging, light leakage due to non-uniform handling of polarized light can lead to cross-talk that degrades the stereoscopic viewing effect. Various solutions have been proposed to compensate for known differences in how coatings respond to s- and p-polarized light. In image projection systems, for example, the use of various types of compensating components, such as quarter-wave retarders, has been taught for correcting the de-polarization of thin-film surfaces.
For projection or display systems that depend inherently on polarization manipulation, such as LCD- or LCOS (liquid crystal on silicon)-based systems, the problem is more acute, since undesired polarization differences and consequent light leakage can directly cause image artifacts. LCDs, of course, modulate polarization orientations temporally and spatially, in a pixel-wise fashion, which means that image quality depends on polarization fidelity. Polarization contributes directly to image contrast (>2000:1 for digital cinema), contrast uniformity, and contrast uniformity with color. There are numerous examples of polarization compensators developed to enhance the polarization performance with LCDs, including those designed for vertically aligned or nematic LCDs. These compensators typically use polymer films to provide angular varying birefringence, structured in a spatially variant fashion, to affect polarization states in portions (that is, within certain spatial and angular areas) of the transiting light beam, without affecting the polarization states in other portions of the light beam. Birefringence is a directional variation of refractive index (Δn=ns−np=nx−ny), that can be provided by intrinsic material properties or by form birefringent sub-wavelength structures. Retardance is the phase change Δφ expressed as distance, where the phase change Δφ(x, t, λ)=2πt(Δn/λ). For example a π/2 (or 90°) phase change Δφ can be provided by a properly oriented compensator having a quarter wave λ/4 of retardance, which, at 550 nm, equals ˜138 nm retardance.
As one example, U.S. Pat. No. 4,701,028 to Clerc et al. describes birefringence compensation designed for a vertically aligned LCD with restricted thickness. U.S. Pat. No. 5,298,199 to Hirose et al. describes the use of a biaxial film compensator correcting for optical birefringence errors in the LCD, used in a package with crossed sheet polarizers, where the LCD dark state has a non-zero voltage (a bias voltage). Additionally, U.S. Pat. No. 5,619,352 to Koch et al. describes compensation devices, usable with twisted nematic LCDs, where the compensators have a multi-layer construction, using combinations of A-plates, C-plates, and O-plates, as needed.
Likewise, in such systems, compensators have also been developed to correct for polarization performance variations of other components, such as the polarization beam splitters or analyzers, either individually or in combination with the LCDs. For example, U.S. Pat. No. 5,576,854 to Schmidt et al. describes a compensator constructed for use in projector apparatus using an LCD with the conventional MacNeille prism type polarization beam splitter. The Schmidt compensator provides 0.27 waves (λ's) of compensation, where 0.25λ's compensate for the MacNeille prism and 0.02λ's of retardance (A-plate) compensate for thermally induced stress birefringence in the counter electrode substrate of the LCD.
In some LCD based projector designs, the color combiner or splitter has its coatings positioned between the LCD panels and the polarization beam splitter. This, in turn, means that system polarization performance is dependent on the design and fabrication of these components, A detailed analysis of this problem is given, for example, by A. E. Rosenbluth et al. in “Correction of Contrast in Projection Systems By Means of Phase-Controlled Prism Coatings and Band-Shifted Twist Compensators” (SPIE Proc., Vol. 3954, pp. 63-90, 2000). Rosenbluth et al. describe a conventional projection architecture in which a “Plumbicon” or “Philips” prism is used in a double-pass configuration to provide both color splitting and recombination. The projector described uses twisted nematic LC panels. The “Plumbicon” tri-prism, described in U.S. Pat. No. 3,202,039 by DeLang, was originally developed for splitting light in TV cameras.
Rosenbluth et al. observe that tilted dichroics are usually polarizing, in that they exhibit a differential phase shift (Δφps=Δφp−Δφs), in both reflection and transmission, which causes polarization mixing, or compound angle depolarization, to transiting light beams. In particular, when oblique or skew light traverses color splitting coatings, which tend to be thick, there are generally differences between the effective penetration depths of the s- and p components, giving the coating a differential phase change Δφps between the two polarizations. Rosenbluth et al. observe that tilted dichroic coatings are usually strong amplitude polarizers at the edges of the band, which in turn implies a strong differential phase shift throughout the band, because of the dispersion integrals that link phase shift with intensity performance. Thus, as s- and p-polarizations transit the coatings at compound incidence angles, they experience different amplitude and phase responses. The most significant polarization cross-talk occurs when skew rays pass through the tilted beam-dividing coatings of the optical system.
Rosenbluth et al. seek to suppress depolarization from the coatings of the tri-prism assembly by designing the prism coatings to collectively remove the rotational component of depolarization rather than designing the coatings to be individually non-phase shifting. While the dispersion integrals link the phase performance and the light throughput efficiency, substantial changes in phase can be provided with marginal effect on throughput. The design of Rosenbluth et al., exploits the double pass geometry of the Philips prism. That is, individual coatings are not designed to be non-phase shifting (zero phase difference, Δφps=0), nor are the coatings designed to be collectively phase corrected in single pass, but they are designed to be collectively phase corrected in double pass. The optimization does not emphasize minimizing amplitude de-polarization effects. Rosenbluth et al. basically designed prism coatings so that skew rays experience elliptical polarization without rotation, while the double-pass symmetry of light passing through the prism assembly advantageously cancels the ellipticity. This symmetry and inherent cancellation only holds if the intensity losses in the dichroic coatings are essentially equal for s- and p-polarizations; thus, correction is imperfect near the band edges. Rosenbluth et al. provide examples in which coating depolarization is optimized in combination with the polarization beam splitter (PBS), with or without the benefit of polymeric film compensators that compensate for PBS or light valve depolarization effects.
Aside from Rosenbluth, the phase implications of multilayer thin-film optical coatings are not widely understood by those who specify and design optical thin-film coatings and have been only briefly explored or noted in the literature. However, few if any of the conventional approaches to this problem achieve satisfactory results.
Because de-polarization can degrade the signal-to-noise (S/N) ratio or contrast in telecommunications and other data signal handling optical systems, there have been a number of proposed solutions for compensating for polarization phase shift Δφ where thin-film surfaces are used for narrow-band optical signals. For example, U.S. Pat. No. 4,373,782 to Thelen entitled “Non-Polarizing Thing Film Edge Filter” describes forming an edge filter having the same s- and p-polarization performance on either the rising or falling edge. This provides a polarization-neutral effect for a single, narrow range of frequencies or wavelengths, but does not compensate for polarization effects beyond this narrow range. Similarly, U.S. Pat. No. 5,579,159 entitled “Optical Multilayer Thin Film and Beamsplitter” to Ito describes coatings design approaches that achieve s- and p-polarized light reflectances that are “substantially close to each other over a predetermined wavelength”. However, the Ito solution is intended for beam splitter use with a single laser beam in the IR range and this method is also not readily extendible beyond a narrow range of wavelengths.
U.S. Pat. No. 6,014,255 entitled “Polarizing Beam Splitter and Magneto-Optic Reading Device Using the Same” to Van Der Wal et al. describes a magneto-optic reader that is optimized to direct light over a single, narrow IR wavelength, with phase difference Δφ reduced by thickening the top and bottom layers over a multilayer thin film design. Again, this method does not address the larger problem of polarization compensation applicable to color-combining optics.
With the increasing use of lasers and other narrow-band light sources, some of the conventional strategies and approaches to the problem of combining modulated light beams, originally developed for more broadband illumination, prove unsatisfactory. Optical filters designed for imaging applications using arc lamp and other broadband sources are typically designed to optimize spectral efficiency, maximize bandwidth, and reject IR or UV light. These components are simply not designed to provide the needed performance over the narrow wavelength bands typical of lasers and other solid state light sources, and are generally designed without regard for polarization phase response.
Conventional approaches to reducing phase differences that affect how multilayer thin-film optical coatings handle polarization may work well enough with narrow-range optical signals for telecommunications. However, these conventional solutions fall short of what is needed to compensate for phase- and polarization-shifting effects of dichroic surfaces in image projection. Thus, there is need for an improved method for design of multilayer thin-film optical coatings for phase difference compensation to provide coatings that exhibit reduced polarization-specific response.